GaN type high electron mobility transistors (HEMTs) that achieve high power operation, high-speed switching, and high-temperature operation have been drawing attention as field-effect transistors (FETs) that may be used for a portable base station and satellite communication, etc. Various techniques for GaN type HEMT are known.
As a source electrode and a drain electrode for a GaN type HEMT, ohmic electrodes are used. FIG. 1 is a sectional view illustrating a GaN type HEMT structure. As illustrated in FIG. 1, a gate electrode 113g and an ohmic electrode 113o are formed over a compound semiconductor layer 110 constituted of GaN material. The ohmic electrode 113o is constituted of a Ta film 111 that is in contact with the compound semiconductor layer 110, and an Al film 112 formed thereover. A Ti film may be used instead of a Ta film 111. Moreover, a SiN film 106 is formed between the gate electrode 113g and the ohmic electrode 113o. Furthermore, an Au film 114 is provided as a wiring over the ohmic electrode 113o through a barrier metal film 120. The reasons why the Au film 114 is used as a wiring are that the resistance is low, allowable current density is high, and related characteristics. The barrier metal film 120 is provided so that the Al film 112 and the Au film 114 do not contact each other. If the Al film 112 and the Au film 114 contact each other, a corrosion reaction called purple plague progresses and the resistance where the two films contact becomes high, and thereby lowers the reliability. Materials for the barrier metal 120 are Ta, Ti, or W, or nitride of these.
However, suppressing the above described corrosion reaction sufficiently and maintaining high reliability are difficult even when the barrier metal film 120 is utilized. Thickening the barrier metal film 120 may be considered in order to suppress the corrosion reaction; however, this may result in a higher resistance value and larger stresses applied around the barrier metal film 120. Related techniques are discussed in Japanese Laid-open Patent Publication No. 2002-359256, and No. 2006-165207.